Traditional spectrometers often use diffractive optics or interferometic devices to resolve the power of the input spectrum at specified wavelengths. The diffractive optical elements or the interferometic devices act as sets of wavelength-specific filters with center frequencies allocated across the wavelengths of interest. Modern high performance spectrometers boast an impressive resolution as high as 0.001 nm in the visible light region. However, most traditional spectrometers are designed for lab use. Their size and cost limits their application, and hence miniature spectrometers have been developed to fill this need. Miniature spectrometers can be manufactured cost effectively by means of micro-electro-mechanical systems (MEMS), micro-optic-electromechanical systems, and/or silicon-bulk techniques.
Common types of miniature spectrometers include grating based, Fabry-Perot based, and Fourier transform based spectrometers. A common purpose among these different types is to make possible a miniature filtering mechanism for separating energy with specific wavelength of interest emitting from an object, such as an object spectrum illustrated in FIG. 1. However, regardless of these different approaches, it is difficult to build high resolution, high quality wavelength-specific filters into miniature spectrometers because of the short optical path (inherent to a miniature spectrometer), the optical properties of silicon IC process compatible materials, and the lack of adequate optical signal conditioning. Consequently, the transmittance properties of the wavelength-specific filters may not be of single-tone, may have large ripples on pass and stop bands, and may have poor stop band attenuation. In these cases, the wavelength-specific filters are highly correlated with each other. From here on, these spectral filters will be called low resolution filters.
The results derived from low resolution filters no longer resemble the spectrum of the input in the wavelength domain. Specifically speaking, when low resolution filters are applied, the output is in terms of a vector space defined by the filters. In other words, the raw output of each spectral filter does not necessarily correspond to a specific wavelength as in traditional spectrometers. For many applications, it is useful and often necessary to view the output in the wavelength domain so as to identify and/or compare the spectral properties of measurement objects.